Detection arrangement and negative impedance for same



June .23, 1942; F TERMAN I 2,287,280

DETECTION ARRANGEMENT AND NEGATIVE IMPEDANCE FOR SAME Filed June 30,1959 vvvvvvvvvv INVENTO R mam-mews TEAMA/V ATTORNEY Patented June 23,1942 DETECTION ARRANGE IMPEDANC Del aware MENT AND NEGATIVE E FOR SAMEFrederick E. Terman, Stanford University, Calii'., assignor to'International Standard Electric Corporation, New York, N. Y., acorporation of Application June so, 1939, Serial No. 282,055

' Claims. (Cl. 178-44) I The present invention relates to improvedcircuit arrangements for performing a detection, and particularly tosuch arrangements wherein a rectifier is used to detect the modulatedcarrier wave so as to produce therefrom a wave corresponding to theenvelope of the modulatedwave.

7 The invention also relates to a novel type of circuit unit having thecurrent/voltage characteristics of a negative admittance, which isemployed in the improved circuit arrangement above mentioned but whichis also suitable for other uses. I

It is an object of the present invention to improve the action ofdetectors, especially detectors of the rectifier type. Especially it isan object to reduce the occurrence .of distortion in such a detector andto enable such detector to be employed for detecting modulated waveshaving a depth of modulation 'approaching 100% without giving rise todistortion or over-modulation. It is a particular object of the presentinvention to raise the alternating current impedance presented to a.diode detector to a value substantially. equal to the value of' thedirect current impedance presented to such detector, in order to causethe diode to faithfully detect modulated signals without alteration ofthe efiective modulation depth, and in order to reduce distortions andpermit the detector to be used with modulated signals having a depth ofmodulation approaching 100%.

In accordance with one feature of the present invention the raising ofthe alternating current impedance presented to adetector of therectifier type by the load circuit of such detector is effected byconnecting a unit having the characteristics of a. negative admittanceacross a portion of such load circuit. It is a further object of thepresent invention to provide an improved type of unit which shallhavethe characteristics of a negative admittance and which shall be suitablefor use not only for raising the effective alternating current impedanceof the load circuit of a detector in accordance with the principalobject of the invention, but also generally forall other uses for whichnegative admittance. units are desired.

In modern radio receivers and other communication devices a common formof detector ar-- rangement for deriving the envelopewave from amodulated wave is the so-called diode detector employing a simple halfwave or full waverectifier of the space discharge type for performwhichcorrespond to the direct This means that the output current tends'to ingdetection. Such diode detectors are genwhich correspond to erally usedina circuit whose impedance' -with respect to direct currents issomewhat higher thanits impedance with respect'to audiofrequency-currents. As a resultof such disparity between the directcurrentand alternating cur- .rent impedances which form the effectiveload for the diode detector, it is usually found that the direct currentoutput from 'the diode does not bear as great a ratio to the audiofrequency current output from the diode as the mean carrier amplitude ofthe high frequency waves ied to the diode bears to the'audiofrequencyamplitude variations of such high frequency waves fed to thediode; s

In other words, because of the smaller impedance offered-to alternatingcurrent in the output circuit of the usual diode detection arrangement,the changes of'carrler amplitude audio frequency signal waves are moreeffectively detected than the unchanging or mean value ofthe=carrier=waves current output from the detector. When, therefore, a wavewhose depth of modulation is equal-toot close to is applied to such adetector, it results that the corresponding output wave of the detectortends to be a pulsating current whosedirect current component is smallerthan its'altemating component as a result of thegreater efl'e'ctjveimpedance of the detector circuit for direct current than for.alternating current.

Because of the unithe diode detector place, and therefore actuallyreverse inpolarity. directional characteristic of such a reversal cannottake a corresponding flattening of the troughsof the audio'frequencyoutput wave necessarily takes place. H

One of the principal objects of 1 the present invention is to eliminatesuch distortion. Briefly this is accomplished by making use of a circuitelement having the'characteristics of a negative admittance for raisingthe-effective alternating current impedance of the load circuit ofadiode detector so that this alternating current impedance shall becomesubstantially equal to'the corresponding direct current impedance ofthe.-

load circuit. r

' The exact nature of the invention may best be understood by referenceto the attached drawing, in whichr Fig. 1 represents a diodedetectorconnected to a circuit arrangement whichserves both'asatwo-stage audio amplifier to amplify the output of the detector,

and also simultaneously serves to present a negative admittance atpoints z-r of the diode output circuit; and

Pig. 2 represents an alternative arrangement wherein the negativeadmittance effect across terminals x's:' is produced by alternativemeans.

Referring more particularly to Fig. l, the diode detector l and itstuned input circuit 2 are connected in series with an output or loadcircuit which primarily includes the resistor I and the resistor l whichis capacitatively coupled' through condenser i. The carrier frequencyby-pass condenser t is also provided across the output circuit in orderto by-pass the carrier frequency. The carrier frequency input to tunedcircuit I is supplied inductively, in the usual manner. Such a circuitarrangement is in itself typical of the conventional circuitsheretoforev impedances N, P. Q, 80 as to form with the impedance-of thedetector circuit a sort of bridge circuit, and in the fact that thecathode lead of the second amplifier tube t! is returned not to groundbut to a point intermediate to .the resistors N and Q.

The three impedances N, P. Q, which are shown as simple resistors inFig. 1, are connected so as r to form with the external impedance Mconnected across the terminals ::a:, a sort of bridge circuit as shown.The symbol M-is used for conwhile the cathode is returned to thediagonally essentially capacitative impedance so that so far as theimpedances presented to the diode are concerned this amplifier might beconsidered as equivalent to a further condenser shunted across theresistor I.

From inspection of the output circuit 1, l, I, I, it will readily beseen that the direct current impedance. of this output circuit .isnecessarily higher than the corresponding alternating current impedancethereof. If it be assumed that resistors 8 and l. are both'of the sameorder of a magnitude and if thecapacity provided by the amplifier acrossterminals :s-cbe neglected, it would follow that thealternatingcurrentimpedance would be approximately one-half of theopposite corner of the bridge which is grounded.

The cathode of this tube tl is given a suitable positive direct currentbias with respect to ground by means of resistor I; Since the controlgrid of tube tlis essentially at ground potential with respect to directcurrent by connection through resistors P and l or Q and N, thispositive cathode bias provided by resistor I has the effect of makingthe grid more negative than the cathode as desired. With respect to alldesired audio frequencies, however, the cathode of tube ii issubstantially short circuited to ground through condenser l so that thiscathode may be regarded as directly grounded with respect to signalfrequencies;

direct current impedance. If any signal more than modulated were appliedto input circuit 2, over-modulation and consequent distortion wouldresult in the output of the diode.

The undesirableratio of direct to alternating current impedance could,it is true, be reduced byzmaking resistance lvery large in comparisonwith resistance 3, but in practice it is usually not possible-to makeresistance 4 higher than a fewmegohms, and since resistance 3 isgenerally at least of the order of a fraction of a megohm.

it is ordinarly not practicable to obtain substantial equality of thealternating current and direct current impedances by simply raisingresistance I. It should furthermore be noted that even if resistance 4were made infinite the alternating current impedance would still besomewhat smaller than the direct currentimpedance because of. thecapacitiveadmittance provided by the wiring and by the input circuit ofthe following audio amplifier.

In accordance with the present invention the audio amplifier not onlydoes not present a capacitive admittance for increasing the disparitybetween direct current and alternating current impedances, but actuallythis amplifier is so con-' nected as to present at the terminals 2-: anegative admittance, thus serving to neutralize the positive admittanceof resistor l. I

The connections of the negative admittance unit shown in Fig.1 to theright of terminals H differ from those of a conventional amplifierprincipally in the connection of the three The suppressor grid oftube tl is directly connected to this cathode and its screen grid-iseffectively short circuited to the cathode with respect to signalfrequencies through condenser 8. Over high resistance II the desireddirect current bias for the screen .grid of tube ti is sup-- plied bybattery I I The plate;of tubetl is fed with the necessary 5 positivedirect current from battery Hover feed-resistor II and coupled overcondenser II and grid resistor It to the control grid of the next tubet2.

The cathode of thesecond tube H is returned to the lower corner ofbridge M, N, P, Q, as shown. Thesuppressor grid of this; tube .t! isdirectly connected to the-cathode and the screen grid isefiectivelyshort circuited to this cathode so'far as the'signal frequenciesconcerned over-condenser It, being fed with the required positivedirectcurrent bias over resistor II from battery II'. The plate of tubeii is fed over impedance II (which is shownas an inductive highimpedance) from battery H. Output coupling condenser 18 couples theplate of tube t! to one of the output terminals o'-o and also to theother comerjof the bridge M, N,' P, Q. The

cathode of tube ii is correspondingly connected to the other of theoutput terminals 0-0 and to V the lower corner-of the bridge M,."N, P,Q. r

The operation of the negative impedance element shown to the'right ofterminals H in Fig. 1, can best be explained by'assuming that anelectromotive force 11- is produced by the action of the diode detector.Not all of this voltage will be effectiveacross theinput of tube tl, butthe drop in N and the drop in Q will-both be effective to. form asignalapplied between ground and the control gridv of tube ti. Thisvoltage between the grid oftube ti and ground will give rise to anamplified voltage acrossthe output of the ampllfler, which outputvoltage will serve to energize the loudspeaker or, other load (notshown) which is connected acrosso-o. This output voltage is'not -onlyeflective across the loudspeaker or other load, ybutisfal so fed back tothe upper and lower corners of the bridge M, N, P, Q. Assuming forconvenience oi. discussion that 7 NP 7 so that M:P=N:Q, then no voltagedrop between the grid of tube ti and groundwillbe produced by such fedback voltage and therefore substantially no degenerationor regenerationwill occur. 'A very considerable current, however, will flow a:'-:rwhich in this case is the diode detector circuit) Such current through Mwill cause a voltage drop in M so that the total voltage across :t-zcwill be the original E. M. REE: plus the added drop due to the currentin M.

It should be noted that the senseor polarityof this current through:c-c; isfso related to the total voltage existing across z :r, as tocorrespond to a negative impedance or more properly a'negativeadmittance on-the ri ht of terminals :ra:. Furthermore, the approximatemagnitude of the negative admittance can .be estimated as followsr Itcan be seen vby inspection thatif the impedance of M- is substantiallygreater than NP/Q so that the bridge M, N, P,,Q, is considerablyunbalanced, then clearly oscillations will occur if the gain oftheamplifier is reasonably great. Since the occurrenceof oscillationsindicates that the admittance of M issmaller in absolute magnitude thanthe negative admittance presented by. the unit-to-the right of, thismeans that for reasonably large values of voltage amplification thevalue of the negative admit-.

tance is only slightly smaller in absolute magnitude than the valueofpositive admittancewhich Mjmust have to balance the bridge M, N, P, Q.In other words, the j admittance presented at 12-: by theback-co'nnected-ampliiler arrangement will be approximately -Q/NP if ais large.

The quantityia is the voltage amplification, i. e. the ratio ofthe'voltage across 00 to the voltage across grid-cathode of tube I underactual operating conditions,

tive admittance oilfered bythe unit the impedance M is assumed to bezero,,or NP/Q, or any other convenient value, and then the ratio of thecurrent through x-a: to the voltage across For more accurate computationof the'nega- ,r-a: is computed for an assumed voltage of Eet If theimpedance of I! be considered; infinite and t e impedances of age, J3,I5, late con-- through the external impedance M (i. e.- through theexternal circuit connected to theterminals pliclty.

the grid otptube ti and" ground arises by virtue of the feedbackvvoltage from cathode of tube t2 tothe upper and lower corners ,of thebridge, still. under-these conditions a lcislight degeneration will bepresent because of' 'In the above discussion certain second orderefiectshave' been neglected for thesakeo t sim Inthe-first place, itshould be-noted.

that even if the bridgeM, N, P, Q, be perfectly balanced'so that nopotential diilerencebetween the plate and the fact that both theigridand 'fllamentoftube t2 7 will be made more positive; thus decreasing.fact that the plate very tube. the degeneration occurring between theoutput the; eilective -platevoltageof 'tube'tlh it should be noted thata slight degeneration-or negative feedback from the output of tube t2 tothe input of this very tubeexists by virtue of the circuit of tube t2includes resistor N while at the same time the input to this tube t2-isappliedfrom' tube I not between the grid and cathode, but" between thegrid of 7 when 'and'ground; The drop in* resistor N f caused bytheoutput current of tube t2 is therefore degeneratively applied to theinput of this It "should be noted, however, that and input of tube tZ-itself maybe considered as 3 equivalent merely to a-reduction in theeffective amplification of this tube and thus is taken into account inthe measured-value 'of a. Similarly the slight degeneration or negative'feedback existing around the whole amplifier from'the out- ,put of tubet2 to the input of'tube ti by virtue of the reduction of efiective platevoltage which occurswhen both theifilament and the grid 'become morepositive is also automatically included in the calculations if a ismeasured under operating conditions;

i For the purpose of eliminating distortion in the diodedetectorcircuit,the best adjustment of the negative admittance presentedby the unit is one which raises the 'alternating'current'im- I pedanceof the diode load circuit to substantially the same valueras the directcurrent impedance of thisload circuit. It is'not desirable toovercompensate so as'to make the altematingcur-v rent impedance greaterthan the direct'current impedance because.when this is the :case theef-'iective' input resistance oil ered by thediodejand its associatedcircuit .to the input wave applied to tuned circuit 2 is greater withrespect to the sideband than with respect to the; carrier. The' resultof this is that whna signal having'a:

depth of modulation-approaching 100% is apsidered zero the negativeadmittance at :r-:r-is I It will be noted that in actual operation Mneed not have the value zeronor the value NP/Q as assumed above, sincethe value of the negative admittance presented at H is independent ofthe external circuit at least if the impedance M of this externalcircuit is not so great asv to cause instability and oscillation. Forvery many purposes, however, it is desired that the negative admittanceof the unit at :t-x should be nearly equal and opposite to the positiveadmittance of the connected circuit. In such cases M will be roughlyequal to NP/Q.

T h to a the ,purely' resistive plied, for example,- by inductivecoupling to the tuned circuit 2, the 'resultinglcarrierfrequency Icurrent flow in tuned circuit'l conta n a larger percentage of. sidebandand a smaller percentage of carrier than the originalmodulated wave, andI thus is likely to correspond tolan over-modulated condition incircuiti.

In the above discussion it-has beenassumedi for the sake of simplicity,that the impedances conductance.

but likewise the shunted capacitive admittance of the wiring, and alsoperhaps or the amplifier circuit itself; r

.I have found that the negative admittance .unit. shown to the right orthe terminals :n-a: in Fig. 1, may be used-not onlyto produce the eiiectof a negative conductance, but generally to produce theiefiectofnegative admittance or a any type.

For this-purpose it ismerely necessary to replace the resistor P by animpedance but also-stray capacities which may exist, acrossthisresistor. 1

Fig.- 2 represents an'alternative arrangement which may in some cases bepreferred, especially when it is desired to improve the direct current,to-alternating current impedance ratio of a diode detector which isalready connected to an existing audio amplifier with-a minimum ofalterations to the existing amplifier. Inthe circuitof Fig. 2, the partscorresponding to parts already described in Fig. l, are designated'bythe same reference characters distinguished by the addition of primes.The apparatus to'the left of the terminals ,.r- :r' is illustrated, forthe sake of generality, as comprising'a diode of-the s c-called fullwave or duplex type rather than a simple half wave dlodejbut actuallysuch apparatus may correspond exactly tothe apparatus to the left of theterminals :c-o: in Fig, 1.- The negative impedance and amplifier unit tothe right'of the terminals :r'-a:' is, however, based upon a differentprinciple from the negative impedance unit shown in Fig. l. The unitshown in Fig. 2 has a positive feedback resistor P' connected from theungrounded side of the output o'o to the ungrounded side of the input:r'--a:'. The connection of such a feedback resistance P tends toproduce a regeneration unless Q has zero impedance, and in order toovercome such an effect a negative feedback 'or degeneration is providedby means of the voltage divider M, N' which serves to make thecathode-oi tube H v more positive at those instants when thepla'te ofsor grid is tied to the cathode and the screen grid is by-passed to thecathode.

The negative admittance across terminals inf-x of the negativeadmittance unit of Fig. 2

where a is large. a Although the inventionis'illustrated-as appliedto adiode rectifier of either the half wave or iull'wave type, it will beunderstood that it i equally applicable to other rectifiers such as, forexample, dry disc rectiflers of the copper oxide or selenium type. Itshould also-be noted ,of desired character such as an-inductance, a

that m y be taken at amt-airt me fromthose indicated In the figures. Ing 1,

for examp1e,'the output as shown, does, not have either side grounded;1,11" desired the output could be taken between the upper ,term-ina'l oand ground in Fig, 1 although in suchcase the impedance ofthe'outputwould have to. be taken into consideration since it would beeffectively across the terminals H.

'Thenegative admittance units of the present inventionareuseiu-l notonlyin connection with a detectionarrangement as shown, but also generallyIorall' otherv applications ,where anegative admittance o1 stablecharacteris desired.

The negative admittance unit of Fig.- 1 (shown to the right of theterminals1:r) may, for example, be'connected directly across. tuned L.0. unit such as the circuit ljfor th'e purpose or neutralizing theresistive admittance of this tuned circuit so as. to raise its effectivereactance resistance ratio. It is well known that an anti-resonantcircuit may be considered equivalent to a pure inductance and purecapacitance and a shunted high resistances Thus the connection or anegative conductance almost equal to the positive conductance of theequivalent shunt resistor will almost neutralize this equivawhole unitwill be correspondingly reactive or complex 50 that the impedance of theunit at :c-a: will be like the impedance at P but with a negative sign.If, for example, P or P is induc tive while the other quantities remainresistive, the negative-admittance presented will correspond to theadmittance of a negative inductance; that is, it will be similarto acapacitance in its phase characteristic, but will become smalleriin'magnitude with rising frequency, thus corresponding to an'inductan'ce'in its frequency characteristic. Although itis preferredto employ P or Pas the generalized impedance for determining theycharacter of thenegative admittance of the units the impedance N or N may be usedinstead of P or? a the controlling impedance. The impedance Q'andM'"also exert control but in inverse fashion. j

Although I prefer to'use, one of the types of negative admittance unitsshown in Figs. 1 or 2, for equalizing the alternating current and directcurrent impedances of a diode detector, other known types of negative,admittance units may be employed. Furthermore, as above mentioned, it isordinarily sufiicient' to employ a negative conductance rather than amore complex negative admittance and, therefore, known types ofdevices'suitable forgiving negative conductance may be employed. V i

Although certain particular embodiments. of my invention have been shownand described for the purposes of illustration; it will be understoodthat various adaptations, alterations and modifications, thereofoccurring to one skilled in the 7 art may be made without departing fromthe scope of my invention as defined in the appended claims.

What I claim is:

'1. A device for presenting to an external circuit a current voltagecharacteristic corresponding to a negative admittance, which comprisesan amplifier having an input including a grid lead and a cathode leadand an output, a first imj pedance joined'in series with said externalcircuit to form a first voltage divider, a connection from the junctionbetween said external circuit and said impedance to the grid lead, asecond and a third impedance joined in series with each other to form asecond voltage divider, a connection from the junction between saidsecond and third impedances to the cathode lead, and means connectingsaid two voltage dividers in parallel across said output.

2. A device for presenting to an external circuit a current voltagecharacteristic corresponding to a negative admittance, which comprises atwostage resistance coupled amplifier having an input and an output,three impedances connected to each other and to said external circuit toform a bridge, connections including a blocking condenser extending fromsaid output to one diagonal of said bridge, and direct connections fromthe other diagonal of said bridge to said input.

3. A device for presenting to an external circuit a current voltagecharacteristic correspond- :0

ing to a negative admittance, which comprises a first and a second spacedischarge tube each having a. cathode and a grid and an anode, means forapplying operating potentials to said elec-' trodes, three impedancesconnected to each other and to said external circuit to form a bridgehaving first and second corners diagonally opposite one another andthird and fourth corners also diagonally opposite one another, aconnection including a blocking condenser extending from the anode ofsaid second tube to the first corner of said bridge, a direct connectionfrom the cathode 01 said second tube to the second corner of saidbridge, two further direct connections from the third and fourth cornersof said bridge to the grid and cathode of said first tube, respectively,and a connection including a second blocking condenser extending fromthe anode of said first tube to the grid of said second tube whereby theanode and cathode of said first tube are coupled to the grid and cathodeof said second tube over said second blocking condenser and oneor saidimpedances.

4. A device according to claim 3, wherein said first and fourth cornersof said bridge are adjacent said external circuit.

7 5. A device according to claim 3, wherein said second and thirdcorners of said bridge are adjacent said external circuit.

